Perception and production of /r/ allophones improve with hearing from a cochlear implant.

Tongue shape can vary greatly for allophones of /r/ produced in different phonetic contexts but the primary acoustic cue used by listeners, lowered F3, remains stable. For the current study, it was hypothesized that auditory feedback maintains the speech motor control mechanisms that are constraining acoustic variability of F3 in /r/; thus the listener's percept remains /r/ despite the range of articulatory configurations employed by the speaker. Given the potential importance of auditory feedback, postlingually deafened speakers should show larger acoustic variation in /r/ allophones than hearing controls, and auditory feedback from a cochlear implant could reduce that variation over time. To test these hypotheses, measures were made of phoneme perception and of production of tokens containing /r/, stop consonants, and /r/+stop clusters in hearing controls and in eight postlingually deafened adults pre- and postimplant. Postimplant, seven of the eight implant speakers did not differ from the control mean. It was also found that implant users' production of stop and stop+/r/ blend improved with time but the measured acoustic contrast between these was still better in the control speakers than for the implant group even after the implant users had experienced a year of improved auditory feedback.

Effects of short- and long-term changes in auditory feedback on vowel and sibilant contrasts.

PURPOSE: To assess the effects of short- and long-term changes in auditory feedback on vowel and sibilant contrasts and to evaluate hypotheses arising from a model of speech motor planning. METHOD: The perception and production of vowel and sibilant contrasts were measured in 8 postlingually deafened adults prior to activation of their cochlear implant speech processors, 1 month postactivation, and 1 year postactivation. Measures were taken postactivation both with and without auditory feedback. Contrast measures were also made for a group of speakers with reportedly normal hearing speaking with masked and unmasked auditory feedback. RESULTS: Vowel and sibilant contrasts, measured in the absence of auditory feedback after 1 month of prosthesis use, were diminished compared with their values measured before prosthesis. Contrasts measured in the absence of auditory feedback after 1 year's experience with the prosthesis were increased compared with their values after 1 month's experience. In both time samples, contrasts were enhanced when auditory feedback was restored. CONCLUSION: The provision of prosthetic hearing to postlingually deafened adults impaired their phonemic contrasts at first, as their auditory feedback had novel characteristics. Once auditory feedback became recalibrated with prosthesis use, it could, in turn, revise feedforward commands that control the contrasts in its absence.

Time course of speech changes in response to unanticipated short-term changes in hearing state.

The timing of changes in parameters of speech production was investigated in six cochlear implant users by switching their implant microphones off and on a number of times in a single experimental session. The subjects repeated four short, two-word utterances, /dV1n#SV2d/ (S = /s/ or /S/), in quasi-random order. The changes between hearing and nonhearing states were introduced by a voice-activated switch at V1 onset. "Postural" measures were made of vowel sound pressure level (SPL), duration, F0; contrast measures were made of vowel separation (distance between pair members in the formant plane) and sibilant separation (difference in spectral means). Changes in parameter values were averaged over multiple utterances, lined up with respect to the switch. No matter whether prosthetic hearing was blocked or restored, contrast measures for vowels and sibilants did not change systematically. Some changes in duration, SPL and F0 were observed during the vowel within which hearing state was changed, V1, as well as during V2 and subsequent utterance repetitions. Thus, sound segment contrasts appear to be controlled differently from the postural parameters of speaking rate and average SPL and F0. These findings are interpreted in terms of the function of hypothesized feedback and feedforward mechanisms for speech motor control.

On the structure of phoneme categories in listeners with cochlear implants.

PURPOSE: To describe cochlear implant users' phoneme labeling, discrimination, and prototypes for a vowel and a sibilant contrast, and to assess the effects of 1 year's experience with prosthetic hearing. METHOD: Based on naturally produced clear examples of "boot," "beet," "said," and "shed" by 1 male and 1 female speaker, continua with 13 stimuli were synthesized for each contrast. Seven hearing controls labeled those stimuli and assigned them goodness ratings, as did 7 implant users at 1-month postimplant. One year later, these measures were repeated, and within category discrimination, d', was assessed. RESULTS: Compared with controls, implant users' vowel and sibilant labeling slopes were substantially shallower but improved over 1 year of prosthesis use. Their sensitivity to phonetic differences within phoneme categories was about half that of controls. The slopes of their goodness rating functions were shallower and did not improve. Their prototypes for the sibilant contrast (but not the vowels) were closer to one another and did not improve by moving apart. CONCLUSIONS: Implant users' phoneme labeling and within-category perceptual structure were anomalous at 1-month postimplant. After 1 year of prosthesis use, phoneme labeling categories had sharpened but within category discrimination was well below that of hearing controls.

Effects of masking noise on vowel and sibilant contrasts in normal-hearing speakers and postlingually deafened cochlear implant users.

The role of auditory feedback in speech production was investigated by examining speakers' phonemic contrasts produced under increases in the noise to signal ratio (N/S). Seven cochlear implant users and seven normal-hearing controls pronounced utterances containing the vowels /i/, /u/, /e/ and /ae/ and the sibilants /s/ and /I/ while hearing their speech mixed with noise at seven equally spaced levels between their thresholds of detection and discomfort. Speakers' average vowel duration and SPL generally rose with increasing N/S. Average vowel contrast was initially flat or rising; at higher N/S levels, it fell. A contrast increase is interpreted as reflecting speakers' attempts to maintain clarity under degraded acoustic transmission conditions. As N/S increased, speakers could detect the extent of their phonemic contrasts less effectively, and the competing influence of economy of effort led to contrast decrements. The sibilant contrast was more vulnerable to noise; it decreased over the entire range of increasing N/S for controls and was variable for implant users. The results are interpreted as reflecting the combined influences of a clarity constraint, economy of effort and the effect of masking on achieving auditory phonemic goals-with implant users less able to increase contrasts in noise than controls.

Effects of bite blocks and hearing status on vowel production.

This study explores the effects of hearing status and bite blocks on vowel production. Normal-hearing controls and postlingually deaf adults read elicitation lists of /hVd/ syllables with and without bite blocks and auditory feedback. Deaf participants' auditory feedback was provided by a cochlear prosthesis and interrupted by switching off their implant microphones. Recording sessions were held before prosthesis was provided and one month and one year after. Long-term absence of auditory feedback was associated with heightened dispersion of vowel tokens, which was inflated further by inserting bite blocks. The restoration of some hearing with prosthesis reduced dispersion. Deaf speakers' vowel spaces were reduced in size compared to controls. Insertion of bite blocks reduced them further because of the speakers' incomplete compensation. A year of prosthesis use increased vowel contrast with feedback during elicitation. These findings support the inference that models of speech production must assign a role to auditory feedback in error-based correction of feedforward commands for subsequent articulatory gestures.

The distinctness of speakers' productions of vowel contrasts is related to their discrimination of the contrasts.

This study addresses the hypothesis that the more accurately a speaker discriminates a vowel contrast, the more distinctly the speaker produces that contrast. Measures of speech production and perception were collected from 19 young adult speakers of American English. In the production experiment, speakers repeated the words cod, cud, who'd, and hood in a carrier phrase at normal, clear, and fast rates. Articulatory movements and the associated acoustic signal were recorded, yielding measures of contrast distance between /a/ and /[see text for symbol]/ and between /u/ and /[see text for symbol]/. In the discrimination experiment, sets of seven natural-sounding stimuli ranging from cod to cud and who'd to hood were synthesized, based on productions by one male and one female speaker. The continua were then presented to each of the 19 speakers in labeling and discrimination tasks. Consistent with the hypothesis, speakers with discrimination scores above the median produced greater acoustic contrasts than speakers with discrimination scores at or below the median. Such a relation between speech production and perception is compatible with a model of speech production in which articulatory movements for vowels are planned primarily in auditory space.

The distinctness of speakers' /s/-/S/ contrast is related to their auditory discrimination and use of an articulatory saturation effect.

This study examines individual differences in producing the sibilant contrast in American English and the relation of those differences to 2 speaker characteristics: (a) use of a quantal biomechanical effect (called a "saturation effect") in producing the sibilants and (b) performance on a test of sibilant discrimination. Twenty participants produced the sibilants /s/ and /S/ in normal-, clear-, and fast-speaking conditions. The degree to which the participants used a saturation effect in producing /s/ and /S/ was assessed with a custom-made sensor that measured contact of the underside of the tongue tip with the lower alveolar ridge; such contact normally occurs during the production of /s/ but not /S/. The acuteness of the participants' discrimination of the sibilant contrast was measured using the ABX paradigm and synthesized sibilants. Differences among speakers in the degree of acoustic contrast between /s/ and /S/ that they produced proved related to differences among them in their use of contact contrastively and in their discriminative performance. The most distinct sibilant productions were obtained from participants who used contact in producing /s/ but not /S/ and who had high discrimination scores. The participants who did not use contact differentially when producing the 2 sibilants and who also discriminated the synthetic sibilants less well produced the least distinct sibilant contrasts. Intermediate degrees of sibilant contrast were found with participants who used contact differentially or discriminated well. These findings are compatible with a model of speech motor planning in which goals for phonemic speech movements are in somatosensory and auditory spaces.

Influences of tongue biomechanics on speech movements during the production of velar stop consonants: a modeling study.

This study explores the following hypothesis: forward looping movements of the tongue that are observed in VCV sequences are due partly to the anatomical arrangement of the tongue muscles, how they are used to produce a velar closure, and how the tongue interacts with the palate during consonantal closure. The study uses an anatomically based two-dimensional biomechanical tongue model. Tissue elastic properties are accounted for in finite-element modeling, and movement is controlled by constant-rate control parameter shifts. Tongue raising and lowering movements are produced by the model mainly with the combined actions of the genioglossus, styloglossus, and hyoglossus. Simulations of V1CV2 movements were made, where C is a velar consonant and V is [a], [i], or [u]. Both vowels and consonants are specified in terms of targets, but for the consonant the target is virtual, and cannot be reached because it is beyond the surface of the palate. If V1 is the vowel [a] or [u], the resulting trajectory describes a movement that begins to loop forward before consonant closure and continues to slide along the palate during the closure. This pattern is very stable when moderate changes are made to the specification of the target consonant location and agrees with data published in the literature. If V1 is the vowel [i], looping patterns are also observed, but their orientation was quite sensitive to small changes in the location of the consonant target. These findings also agree with patterns of variability observed in measurements from human speakers, but they contradict data published by Houde [Ph.D. dissertation (1967)]. These observations support the idea that the biomechanical properties of the tongue could be the main factor responsible for the forward loops when V1 is a back vowel, regardless of whether V2 is a back vowel or a front vowel. In the [i] context it seems that additional factors have to be taken into consideration in order to explain the observations made on some speakers.

Economy of effort in different speaking conditions. I. A preliminary study of intersubject differences and modeling issues.

This study explores the hypothesis that clear speech is produced with greater "articulatory effort" than normal speech. Kinematic and acoustic data were gathered from seven subjects as they pronounced multiple repetitions of utterances in different speaking conditions, including normal, fast, clear, and slow. Data were analyzed within a framework based on a dynamical model of single-axis frictionless movements, in which peak movement speed is used as a relative measure of articulatory effort (Nelson, 1983). There were differences in peak movement speed, distance and duration among the conditions and among the speakers. Three speakers produced the "clear" condition utterances with movements that had larger distances and durations than those for "normal" utterances. Analyses of the data within a peak speed, distance, duration "performance space" indicated increased effort (reflected in greater peak speed) in the clear condition for the three speakers, in support of the hypothesis. The remaining four speakers used other combinations of parameters to produce the clear condition. The validity of the simple dynamical model for analyzing these complex movements was considered by examining several additional parameters. Some movement characteristics differed from those required for the model-based analysis, presumably because the articulators are complicated structurally and interact with one another mechanically. More refined tests of control strategies for different speaking styles will depend on future analyses of more complicated movements with more realistic models.

Economy of effort in different speaking conditions. II. Kinematic performance spaces for cyclical and speech movements.

This study was designed to test the hypothesis that the kinematic manipulations used by speakers in different speaking conditions are influenced by kinematic performance limits. A range of kinematic parameter values was elicited by having seven subjects produce cyclical CV movements of lips, tongue blade and tongue dorsum (/ba/, /da/, /ga/), at rates ranging from 1 to 6 Hz. The resulting measures were used to establish speaker- and articulator-specific kinematic performance spaces, defined by movement duration, displacement and peak speed. These data were compared with speech movement data produced by the subjects in several different speaking conditions in the companion study (Perkell et al., 2002). The amount of overlap of the speech data and cyclical data varied across speakers, from almost no overlap to complete overlap. Generally, for a given movement duration, speech movements were larger than cyclical movements, indicating that the speech movements were faster and were produced with greater effort, according to the performance space analysis. It was hypothesized that the cyclical movements of the tongue and lips were slower than the speech movements because they were more constrained by (coupled to) the relatively massive mandible. To test this hypothesis, a comparison was made of cyclical movements in maxillary versus mandibular frames of reference. The results indicate that the cyclical movements were not strongly constrained by mandible movements. The overall results generally indicate that the cyclical task did not succeed in defining the upper limits of kinematic performance spaces within which the speech data were confined. Thus, the hypothesis that performance limits influence speech kinematics could not be tested effectively. The differences between the speech and cyclical movements may be due to other factors, such as differences in speakers' "skill" with the two types of movement, or the size of the movements--the speech movements were larger, probably because of a well-defined target for the primary, stressed vowel.